1. Field of the Invention
The present invention relates to composite materials which are capable of storing thermal energy when exposed to heat, sunlight and/or electricity.
2. Description of the Prior Art
A series of organic polyols are related compounds with tetrahedral molecular structures has been found to be suitable for thermal energy storage. These polyols include pentaerythritol (C.sub.5 H.sub.12 O.sub.4), pentaglycerine (C.sub.5 H.sub.12 O.sub.3), neopentyl glycol (C.sub.5 H.sub.12 O.sub.2), neopentyl alcohol (C.sub.5 H.sub.12 O) and neopentane (C.sub.5 H.sub.12). Certain of these polyols can be alloyed together to provide reversible solid-solid mesocrystalline phase transformations of high enthalpy and adjustable temperatures of transition.
These polyols are referred to as phase change materials (PCMs), which reversibly absorb large amounts of thermal energy during solid-state transformations at temperatures well below their melting temperatures. These transformation temperatures may be adjusted over a wide range by selecting various compositions of solid-solution mixtures of the polyols.
A large number of phase change materials were evaluated by NASA in the 1960's as "thermal capacitors" to passively buffer the temperature swings experienced by earth orbiting satellites. See Hale et al., Phase Change Materials Handbook, NASA Report B72-10464 (August 1972). Among the hundreds of phase change materials evaluated by NASA were a few materials which exhibited solid-to-solid transformations with large enthalpies. Though these materials were not used for space applications, a decade later they became of interest to scientists searching for better phase change materials for thermal energy storage. Solid-state phase change materials have the advantages of less stringent container requirements and greater design flexibility.
Energy is commonly stored in heated bricks, rock beds, concrete, water tanks, and the like. Such thermal energy storage methods require leakproof containers and/or extensive space and mechanical support for the massive amounts of storage materials. In such materials, the amount of energy stored is proportional to the temperature rise and to the mass of the storage material, and is generally limited to about 1 calorie per gram per .degree.C. (1 BTU per pound per .degree.F.).
In contrast, phase change materials store much larger amounts of thermal energy over a small temperature change by virtue of reversible physical/chemical/structural changes such as melting. For example, certain hydrated inorganic salts used for thermal energy storage absorb as much as 96 BTU per pound at the melting temperature.
However, there are disadvantages to the use of solid/liquid phase change materials. They must be reliably maintained in a durable container and their melting-crystallization change must be fully reversible. In the past, many solid/liquid phase change materials have leaked and/or have lost storage capacity because of irreversible changes over periods of time. In addition, the conduction of heat into and out of solid/liquid phase change materials is commonly limited by the poor thermal properties of the liquid phase of the material and/or its interface with the container used to hold the phase change material.
In general, the thermal conductivity of a phase change thermal storage material is an important parameter, as well as its transition temperature. To a certain extent, the thermal conductivity of phase change materials is adjustable by introducing additives to form composite materials. For example, the heat transport in paraffin phase change materials can be adjusted by introducing metal matrices, such as aluminum honeycomb or expanded aluminum mesh into the phase change material container. See: deJong, A.G. Improvement of Heat Transport in Paraffins for Latent Heat Storage Systems, in Thermal Storage of Solar Energy (C. den Ouden, ed.) pp. 123-1344 (1981); and Benson et al., Solid State Phase Change Materials for Thermal Energy Storage in Passive Solar Heated Buildings, Proceedings of the Tenth Energy Technology Conference, Washington, D.C., pp. 712-720, (Feb. 28-Mar. 2, 1983). Other literature discusses a class of hydrocarbon compounds for use in thermal energy storage components for passive solar heated buildings, with particular reference to polyhydric alcohols such as pentaerythritol, trimethylol ethane, neopentyl glycol, and closely related materials. This work also discusses solid-state phase change materials which provide compact thermal energy storage with reduced concern for the containment of the phase change material. Another work Christensen, Advanced Phase Change Storage for Passive Solar Heating: Analysis of Materials and Configurations, in Proceedings of the ASES Passive 83 Conference, Glorieta, N.M., (Sept. 7-9, 1983) discusses the performance of phase change materials for thermal storage in passive solar heating systems, including factors other than material properties that affect storage performance and optimization.
A related work Benson et al. Materials Research for Passive Systems-Solid State Phase Change Materials and Polymer Photodegradation, in Proceedings of the Passive and Hybrid Solar Energy Update, Washingston, D.C., pp. 228-235, (Sept. 15-17, 1982), discusses solid-state phase change materials being evaluated for use in passive solar thermal energy storage systems, with particular emphasis on pentaerythritol, pentaglycerine and neopentyl glycol. Another work, Benson, Organic Polyols: Solid State Phase Change Materials for Thermal Energy Storage, in Opportunities in Thermal Storage R and D, EPRI Special Report EM-3159-SR, pp. 19-1 to 19-10 (July 1983); discusses a homologous series of organic polyols based on pentaerythritol, including pentaglycerine and neopentyl glycol, demonstrating potential for thermal energy storage at temperatures from below 25.degree. C. to 188.degree. C.